Coupling of two or more laser beams often becomes necessary if a beam from a single laser cannot provide sufficient power (if the beam is CW beam), sufficient average power (if the beam is a beam from a repetitively pulsed laser), or sufficient peak power (if the beam is repetitively-pulsed beam or an individually-pulsed beam) for a particular application.
Beam combining methods may be summarized as being of two different types. One type, which is often referred to as temporal interleaving, is used exclusively for combining beams from repetitively pulsed lasers. This type of combining is useful in applications in which an increased average power in a sequence of pulses is as advantageous, or preferable to, increasing the peak of pulses in a sequence thereof. In this method, beams from two or more repetitively-pulsed lasers having the same pulse repetition frequency (PRF) but having a phase difference therebetween are combined along a common path to provide a combined beam having two or more times the PRF of the individual beams and an average power which is about the sum of the average powers of the individual beams. Such a method is described in U.S. Pat. No. 5,999,555.
Temporal interleaving cannot be used when it is desired to combine beams either for increasing CW power or for increasing peak power in a pulse or a repeated sequence of pulses. Spatial combining methods have been used for this type of combination. Prior-art spatial beam combining methods include combining two plane-polarized beams which have orthogonally oriented polarization planes using a polarization-dependent transmissive and reflective beam combiner such as a McNeill biprism. This method however, is practically limited to combining only two beams, and is not practical for combining unpolarized beams. Methods for combining more than two beams include combining beams having slightly different wavelengths using a diffraction grating, prism or dichroic mirror. This method is only useful however in applications that are insensitive to the bandwidth of radiation. Another method of combining two or more beams is simply to cause the beams to propagate at an angle to each other and overlap in a common area in a plane in which a substrate being treated by the combined beams is located. This may be termed oblique coupling and the beams are only coupled at the location at which they intersect.
In most applications requiring a plurality of laser beams to be coupled into a single beam or into a common area, whatever coupling method is employed, it is important, at least, that the light-intensity distribution across the coupled beams not be influenced by variations in the intensity distribution in individual laser beams. This is particularly true in very high-power excimer-laser-illuminated photomask (mask) projection systems used in material processing applications, or in optical lithography applications used in semiconductor device manufacture. One step that has been proposed to at least partially achieve such a result is to subject each of the beams to be combined to a beam homogenization step, before the beams are coupled. This is disclosed in detail in German Patent Publication No. DE10301482, and is described briefly below with reference to FIG. 1.
FIG. 1 is substantially reproduced from the German '482 publication, and depicts a photomask imaging (projection) apparatus 10 in which a beam 12 from one laser, and beam 14 from another laser, initially propagating toward each other, are caused to propagate parallel to each other by mirrors 16 and 18, respectively. Beam 12 is homogenized with the aid of a homogenizer 20A including the cylindrical lens arrays 22 and 24. Beam 12 is incident on cylindrical lens array 22 and cylindrical lenses 23 in the array divide the beam into a plurality of ray-bundles 26 with rays in each bundle initially converging. Ray bundles pass through an intermediate focus from which rays in the bundles diverge, and are incident on corresponding cylindrical lenses 25 of cylindrical lens array 24. Beam 14 undergoes this same division and optical operation in a homogenizer 20B identical with homogenizer 20A.
Bundles 26 are individually divergent on exiting cylindrical lens array 24 and propagate parallel to each other. The bundles are intercepted by a collecting or condenser lens 28 arranged on an optical axis 11. Homogenizers 20A and 20B are disposed symmetrically on opposite sides of axis 11. Lens 28 reduces the individual divergence of bundles 26 and causes the beams to be mutually convergent, causing the bundles to intersect in a plane 34 (mask plane or object plane) in which a photomask (object) would be located. A field lens 32 close to the mask plane further reduces the individual divergence of the bundles enabling them to pass the entrance pupil of the projection lens 38.
Because of the symmetrical arrangement of the homogenizers about axis 11, ray bundles from homogenizer 20B also intersect with each other, and with ray bundles from homogenizer 20A, such that, in plane 34, homogenized beams 12 and 14 are effectively coupled and the intensities of the beams are summed. Object points 36A, 36B, and 36C in plane 34 are imaged by a projection lens 38 into an image plane 40 in which a substrate would be located to receive a mask image, as is known in the art. Only one image point 42B (an on-axis image point) is depicted in FIG. 1 for economy of illustration.
It has been determined that in the above-described beam coupling apparatus the homogenization of the beams prior to coupling is insufficient in itself to provide uniform, temporally-constant illumination in image plane 40. This is believed to be a result of one or more of the following problems.
As may be seen in FIG. 1, ray pencils originating from homogenizers 20A and 20B (laser 1 and laser 2) are incident in mask plane 34 at different angular directions, and consequently fill complementary portions of the projection cone converging on the image plane 40, as exemplified for image point 42B. Only under perfect imaging conditions, for example diffraction limited, and, in particular, if image plane 40 and the substrate surface perfectly coincide, will the image power distribution be independent of the angular distribution.
Under practical conditions, some deviation from this perfect condition has to be reckoned with, for example, because of residual field curvature of the projection lens 38, or because of dynamic variation of the substrate plane position with respect to the image plane 40 during high speed scanning operation. In order to counter these deviations, the projection lens is typically designed to be telecentric, with the tacit assumption of angular symmetric illumination.
However, if, for example, the powers of the beams leaving the two homogenizers 20A and 20B differ, or in the case of interleaving laser operation, the illumination cones for each image point will show an asymmetric angular power distribution, and the telecentricity of the image projection is impaired, giving rise to variations of the image power distribution (shot-to-shot variations) under practical operation conditions. Another problem, albeit less severe is as follows
The illumination provided by any one beam in plane 34 is asymmetric because of the non-symmetrical oblique incidence of rays in the plane. Rays from beam 14, incident in plane 34, are shown in dashed lines to highlight this. The coupling method relies on there being an exactly complimentary asymmetry in one beam compared with the other. Accordingly, if the power in the two beams is different there will be some residual asymmetry of illumination in plane 34 in which the beams are coupled.
Precise intersection of the beams requires that the pointing of beams to be constant. Unfortunately, in high-power lasers of any kind, this is rarely the case. Pointing varies temporally, and differences in pointing in beams 12 and 14 may lead to poor definition and variable illumination at edges of the area illuminated in plane 34. Finally, but not exhaustively, when the beams are delivered by pulsed lasers, the pulses in each beam must preferably be synchronized to at least consistently, if not exactly, overlap in time. In excimer laser beams, where pulses may have a duration as short as about 20 nanoseconds (ns) or less, an exact or consistent temporal overlap is very difficult to achieve. This can lead, for example, to illumination in plane 34 having, at the leading edge of the pulse, primarily the characteristics of one laser beam, and at the trailing edge of the pulse the characteristics of the other laser beam.
Clearly, while the above-discussed problems are discussed in the context of one particular method of beam coupling at least one of the problems would be encountered in other beam coupling methods. Accordingly, there is a need for a method of solving or at least mitigating these problems to expand the use of beam coupling methods for delivering high laser power or laser energy.